Low-pressure plasma systems, which work with plasma excitation frequencies of more than 1 MHz in the high-frequency (HF) range that is relevant here, are used, e.g., for cleaning or activating objects made of metal, plastic, glass, and ceramic before further processing, such as, for example, before sputtering, spray painting, cementing, printing, soldering, etc., for etching or coating by means of plasma, for plasma generation for gas lasers, e.g., CO2 lasers, and for many other applications.
John R. Hollahan and Alexis T. Bell: “Techniques and Applications of Plasma Chemistry”, published by John Wiley & Sons, Inc. (1974), pp. 393-399, describe a widely used conventional low-pressure plasma system. This low-pressure plasma system contains a vacuum chamber to hold a medium or process gas to be ionized, a vacuum pump to produce a vacuum in the chamber, a high frequency generator, and means to apply a high frequency field to the vacuum chamber and an impedance matching network. It can possibly also contain means to measure the HF power supplied to the vacuum chamber and/or means to measure its pressure.
The high frequency (HF) generator supplies the HF power required for generating the plasma, said HF power having values between, e.g., 10 W and a few kW at the high operating frequency. This high frequency generator must meet the regulations relating to the usable high frequency bands, in particular the frequencies of 6.78 MHz, 13.56 MHz, 27.12 MHz, or 40.68 MHz that are allocated for industry, science and medicine (ISM), and it must be able to operate with an output impedance of 50 ohms. Such an HF generator generally contains a crystal-controlled HF oscillator, which usually works with an LC oscillating circuit or as a feedback oscillator at a resonant frequency that corresponds to the desired ISM frequency, and serves as a source for the HF signal. The HF generator also has an amplifier, which amplifies the signal from the oscillator, and an output stage that is designed to provide a desired output power and a fixed output impedance of the HF generator. As a rule, this is accomplished by providing a circuit consisting of coils and capacitors with fixed inductance and capacitance values, which transforms the impedance at the output of the amplifier into the desired output impedance of the HF generator, e.g., 50 ohms. The HF power signal generated by the HF generator is transferred through leads and applied to electrodes of the vacuum chamber to excite the process gas therein, to cause an electric discharge, and in this way to generate a plasma.
The impedance matching network, which is also called a matching unit, is connected between the HF generator and the vacuum chamber and serves to match the impedance of the generator and that of the load, in particular the plasma chamber with the plasma located therein. If the impedance is well matched, power from the generator can be transferred with high efficiency until the discharge in the vacuum chamber. By contrast, if the impedance is mismatched, there are reflections in the lead between the HF generator and the load, which have the consequence of power losses. Impedance matching circuits are often based on an L, T, or Pi type arrangement of coils and capacitors, which are to be suitably dimensioned.
In a low-pressure plasma system, the load impedance depends on numerous factors and is variable. For example, the impedance depends on the geometric shape of the vacuum chamber and the arrangement of the electrodes in the vacuum chamber. The type of the ionizable medium or process gas used, the pressure and the temperature in the vacuum chamber, and the supplied power also affect the impedance. The impedance changes suddenly on ignition, and also does not remain constant in operation. The line impedance of the lead between the HF generator, which is frequently remotely located, and the vacuum chamber must also be taken into consideration. Therefore, low-pressure plasma systems typically use variable impedance matching networks, whose coils and capacitors have variable inductance and capacitance values and are adjustable, to transform a load impedance into the 50 ohm output impedance that is typical of an HF generator. However, suitable and optimal adjustment of the coils and capacitors to match the impedance is laborious and time-consuming, depends greatly on the experience of the operator, and is hardly reproducible. Frequently, the power output by the HF generator is compared with the power measured at the vacuum chamber, and the coils and capacitors are adjusted by testing, to match the powers. Without correct matching, there can be difficulties when the process gas breaks down in the vacuum chamber. In rare cases, after ignition the plasma can be extinguished again, due to a decrease in impedance. Optimal impedance matching can represent an extremely challenging and laborious procedure, even for experienced specialized personnel.
DE 39 42 560 C2 describes a high frequency generator for a plasma-generating consumer with a direct voltage energy source and a high-frequency actuated electronic switch, wherein the plasma-generating consumer is connected directly between the energy source and the switch, without interposition of an impedance matching network. The switch is actuated by a square wave HF control signal, alternately turning on and off the output voltage of the energy source, and this output voltage is applied directly to the plasma-generating consumer as an operating voltage. To allow the square wave voltage to be transferred, this HF generator deliberately does without inductors and capacitors between the energy source and the terminals of the generator. Moreover, at supply voltages of some hundred volts, the MOSFET must be able to switch high currents on the order of 10 amperes or more in a few nanoseconds, and it must have low self-inductance. Therefore, special fast switching power MOSFET transistors are used here, which have internal connections each of which is connected in an inductance-reducing manner with external connections of the housing of the MOSFET transistor through multiple bond wires, the housing having two gate-side source connections and two drain-side source connections and each of the connections of the housing having a so-called stripline structure, i.e., being realized outwardly flat and closely adjacent. Such fast switching power MOSFET transistors are absolutely necessary in this circuit topology, however they are relatively large in size, are very costly, and require a correspondingly complex circuit environment and especially cleverly thought-out, fast, and costly gate control units (gate drivers). It is desirable to use switches and associated drive units that are relatively commercially available and economical even for HF low-pressure plasma applications.
DE 33 37 811 A1 discloses a high frequency generator that has a direct voltage energy source whose output is connected with a controllable electronic switch. The direct voltage that is turned on and off by the switch is fed to an energy buffer and transfer device in the form of a transformer, whose output is connected with a low-temperature plasma generator. The electronic switch is controlled in a pulse-like manner by a control circuit, producing, at the output of the transformer, a high-voltage output pulse to operate the plasma generator. This generator cannot generate high-frequency high-voltage pulses, i.e., pulses in the MHz range, since at such input frequencies the transformer can no longer transfer or generate high-voltage pulses with such frequencies at its output as a consequence of its ohmic and inductive and capacitive resistance. Moreover, here the switch, a MOSFET, is connected between the energy source and the transformer, so that the source potential forms a floating reference potential for the gate terminal and the control circuit connected to it. Therefore, here especially low-capacitance power supply is required, to avoid high switching losses, EMC problems, and possible malfunctions. This additionally makes this generator unsuitable for use in HF low-pressure plasma systems.